1. Field of the Invention
The present invention relates to a solid state radiation detector with a charge storage portion for storing an electric charge with a quantity corresponding to the dose of radiation irradiated, an image recording method and an image recorder for recording radiation image information on the charge storage portion as an electrostatic latent image by the use of the detector, and an image reading method and an image reader for reading out the recorded electrostatic latent image from the detector.
2. Description of the Related Art
Units, which employ an image detector, such as a facsimile, a copying machine, a radiation imaging unit, etc., are known.
For example, in the medical radiation imaging unit, for the purposes of a reduction in the radiation dose to which a subject is exposed, an enhancement in diagnosis performance, etc., a solid state radiation detector (electrostatic storage) with a photoconductor (layer) such as a selenium plate sensitive to radiation such as X-rays is employed as an image detector. X-rays are irradiated to the solid state radiation detector to store an electric charge with a quantity corresponding to the dose of the irradiated radiation in the charge storage portion of the detector as latent image charge. With the irradiation, radiation image information is recorded on the charge storage portion as an electrostatic latent image. The solid state radiation detector recorded with the radiation image information is scanned by a laser beam or a line light source to read out the radiation image information from the detector.(e.g., Japanese Unexamined Patent Publication No. 6(1994)-217322, U.S. Pat. No. 4,857,723, Japanese Unexamined Patent Publication No. 9(1997)-5906, etc.).
The method as described in the above-mentioned Japanese Unexamined Patent Publication No. 6(1994)-217322 uses, as an image detector, a solid state radiation detector in which (1) a conductive layer, (2) an X-ray photoconductive layer, (3) a dielectric layer, and (4) an electrode layer with a plurality of micro plates corresponding to pixels are stacked and also having a thin film transistor (TFT) connected to each micro plate. X-rays transmitted through a subject are irradiated to the solid state radiation detector to store latent image charge in a charge storage portion formed between each micro plate and the conductive layer. With the storage, the radiation image information is recorded in the solid state radiation detector. Next, the TFTs are scanned to read out the latent image charge stored in the charge storage portion from the solid state radiation detector. In this manner the radiation image information is read out from the solid state radiation detector.
The method as described in the above-mentioned U.S. Pat. No. 4,857,723 uses, as an image detector, a solid state radiation detector constructed so that both sides of a photoconductive layer are interposed between insulating layers and that the outsides of the insulating layers are interposed between stripe electrodes which have a plurality of line electrodes crossing one another. X-rays transmitted through a subject are irradiated to the solid state radiation detector to store latent image charges of opposite polarities in two charge storage portions formed at the interface between the photoconductive layer and the insulating layer, at the positions where both stripe electrodes cross. After the radiation image information is recorded in the solid state radiation detector, it is scanned with laser light (reading light) to read out the latent image charges stored in the charge storage portion from the solid state radiation detector. In this way the radiation image information is read out from the detector.
In the method as described in the above-mentioned Japanese Unexamined Patent Publication No. 9(1997)-5906, a solid state radiation detector, in which a trapping layer (first electrode layer), a reading photoconductive layer (recording photoconductive layer), and a second electrode layer (charge storage portion) are stacked in the recited order, is used as an image detector. With high voltage applied between electrodes disposed on both sides of the detector, uniform exposure light is irradiated to cause primary charging in the charge storage portion of the detector. Thereafter,both electrodes are short-circuited, or given high voltage, or made open circuit. With an electric field generated in the recording photoconductive layer, X-rays transmitted through a subject are irradiated to store latent image charge in the charge storage portion. After the radiation image information is recorded in the solid stage radiation detector, both electrodes are short-circuited. The solid stage radiation detector is scanned with laser light (reading light) to read out the latent image charge stored in the charge storage portion from the solid stage radiation detector. Thus, the radiation image information is read out from the detector.
However, method as described in the above-mentioned Japanese Unexamined Patent Publication No. 6(1994)-217322 has the disadvantage that the image detector is structurally complicated and the fabrication cost becomes high, because TFTs for reading electric charge are provided within the electrode layer equipped with micro plates.
In addition, the method as described in the above-mentioned U.S. Pat. No. 4,857,723 is a method in which the solid state radiation is structurally simple and the fabrication cost is low, but requires a structurally complicated expensive layer scanning system for reading out the latent image charge stored within the detector. Because of this, the method has the disadvantage that the reader is structurally complicated and the cost of the entire system from recording to reading becomes high.
Furthermore, the method as described in the abovementioned Japanese Unexamined Patent Publication No. 9(1997)-5906, in addition to the disadvantage found in the method described in the above-mentioned U.S. Pat. No. 4,857,723, requires a light source for causing primary charging in the charge storage portion of the detector, so it has the disadvantage that the detector is further increased in scale and the system costs become even higher.
It is the primary object of the present invention to provide an image detector, a recording method and a recorder for recording radiation image information in the image detector, and a reading method and a reader for reading out radiation image information from the image detector recorded with the radiation image information, which are capable of making its structure simple without using a thin film transistor, also reducing the fabrication cost, and making a simple read operation possible without using a light source for primary charging, or reading.
In accordance with the present invention, there is provided an image detector, which has a charge storage portion for storing an electric charge with a quantity corresponding to the quantity of a recording electromagnetic wave irradiated, for recording image information on the charge storage portion as an electrostatic latent image, the image detector comprising:
a first electrode layer with a first stripe electrode including a large number of line electrodes;
a photoconductive layer which exhibits photoconductivity when irradiated with the recording electromagnetic wave, and/or with light, emitted by excitation of the electromagnetic wave, which has a wavelength differing from the wavelength of the electromagnetic wave;
a rectifying layer; and
a second electrode layer with a second stripe electrode including a large number of line electrodes formed so as to cross the line electrodes of the first stripe electrode;
wherein the first electrode, the photoconductive layer, the charge storage portion, the rectifying layer, and the second electrode layer are stacked in the recited order.
The aforementioned recording electromagnetic wave refers to an electromagnetic wave incident on the detector. The electromagnetic wave will be sufficient if it carries image information. For instance, radiation such as X-rays, etc., or light (not limited to visible light) can be employed. It may also be light, emitted by excitation of a primary electromagnetic wave (radiation, light) emitted from an electromagnetic wave source, which has a wavelength differing from the wavelength of the primary electromagnetic wave.
The aforementioned line electrode refers to a long and narrow electrode. It may have any shape such as a column shape, a square pillar shape, etc., as long as it is long and narrow. Particularly, a flat electrode is preferred. When the line electrodes of the first stripe electrode are disposed so as to cross the line electrode of the second stripe electrode, it is preferable to dispose the line electrodes so that they cross approximately perpendicularly.
The aforementioned rectifying layer means a layer which allows a current to flow in one direction and prevents most of the current from flowing in the other direction.
The rectifying layer is generally constructed of an n-type semiconductor, an insulator, and a p-type semiconductor. However, depending on materials (e.g., xcex1-Si, etc.) constituting these semiconductors, resistivity is small and there is a possibility that charge-transfer will occur within the p-type or n-type semiconductor and constitute a hindrance to image formation. Therefore, in the case where the charge-transfer cannot be allowed, the rectifying layer constituting the detector of the present invention is formed by disposing an n-type semiconductor, an insulator, and a p-type semiconductor in the recited order. One of the n-type and p-type semiconductors, which is disposed on the side of the charge storage portion, is divided two-dimensionally so as to correspond to pixel positions defined by the first and second stripe electrodes. Also, one of the n-type and p-type semiconductors, which is disposed on the side of the second electrode layer, is divided in stripe form (one-dimensionally) so as to correspond to the second stripe electrode, or divided two-dimensionally so as to correspond to the pixel positions. That is, it is preferable that the semiconductors be divided in both directions where both stripe electrodes are arrayed.
The expression xe2x80x9cpixel positions defined by the first and second stripe electrodesxe2x80x9d means spatial positions between both stripe electrodes, defined when both stripe electrodes cross three-dimensionally. The spatial positions do not always accurately coincide with the above-mentioned crossing positions, but in some cases move slightly, depending on a method of forming the charge storage portion. For example, when the charge storage portion is formed by conductive members to be described later, the pixel positions are subjected to the influence of the positions where the conductive members are disposed.
To divide the p-type and n-type semiconductors in the aforementioned manner, etching may be utilized. The image detector in this form will hereinafter be referred to as an image detector in which the rectifying layer is divided according to pixel positions.
Note that the insulator interposed between the n-type semiconductor and the p-type semiconductor may also be divided in stripe form (one-dimensionally) so as to correspond to the first or second stripe electrode, or divided two-dimensionally so as to correspond to the aforementioned pixel positions.
In the image detector of the present invention, it will be sufficient if the charge storage portion for storing latent image charge is formed inside the photoconductive layer or near the inside of the layer. Preferably, the charge storage portion is formed by conductive members (micro plates) which are provided separately in an electrically non-connected (floating) state for each of the pixel positions and cause the aforementioned electric charge to be in the same electric potential state.
In the image detector of the present invention, it is preferable to provide a scintillator on the outside of the first electrode layer. The scintillator emits light having a wavelength differing from the wavelength of the recording electromagnetic wave, by excitation of the electromagnetic wave.
It is desirable that the rectifying layer of the image detector of the present invention have permeability with respect to the recording electromagnetic wave, or with respect to light, emitted by excitation of the recording electromagnetic wave, which has a wavelength differing from the wavelength of the electromagnetic wave.
To make the rectifying layer have permeability, it is formed from a material that has permeability with respect to the recording electromagnetic wave, or with respect to light, emitted by excitation of the recording electromagnetic wave, which has a wavelength differing from the wavelength of the electromagnetic wave.
The rectifying layer may include a large number of rectifying devices formed so that the size of each device is two-thirds or less of the smallest resolvable pixel size. Also, spaces between the rectifying devices may be filled with a material that has permeability with respect to the recording electromagnetic wave, or with respect to light, emitted by excitation of the recording electromagnetic wave, which has a wavelength differing from the wavelength of the electromagnetic wave.
In the case where the insulator interposed between the n-type semiconductor and the p-type semiconductor is divided in stripe form (one-dimensionally) so as to correspond to the first or second stripe electrode, or divided two-dimensionally so as to correspond to the aforementioned pixel positions, as described above, the entire rectifying layer is divided so as to correspond to the stripe electrode or the aforementioned pixel positions, and light is transmitted through between the divided members. In the case of the detector in this form, the rectifying layer practically has permeability with respect to the recording electromagnetic wave, or with respect to light, emitted by excitation of the recording electromagnetic wave, which has a wavelength differing from the wavelength of the electromagnetic wave.
In the image detector of the present invention, it is more desirable to provide a scintillator on the outside of the second electrode layer. The scintillator emits light having a wavelength differing from the wavelength of the recording electromagnetic wave, by excitation of the electromagnetic wave.
In accordance with the present invention, there is provided a method of fabricating the image detector in which the rectifying layer is divided according to pixel positions, the method comprising the steps of:
forming a second electrode layer;
forming one of n-type and p-type semiconductors on the second electrode layer and then etching the one semiconductor so that it is disposed facing a second stripe electrode;
forming an insulator on the etched one semiconductor;
forming the other of the n-type and p-type semiconductors on the insulator, and then etching the other semiconductor so that it faces the second stripe electrode and has desired pixel pitches and pixel widths in the longitudinal direction of the second stripe electrode;
forming a charge storage portion and a photoconductive layer on the etched other semiconductor in the recited order; and
forming an electrode member, which forms a first electrode layer, on the photoconductive layer and then etching the electrode member in stripe form so that the first stripe electrode is disposed facing a position where the etched other semiconductor is disposed. Note that the second electrode layer may be formed on a photoconductive support member such as a glass substrate, etc.
In the fabrication method of the present invention, the aforementioned one semiconductor of the n-type and p-type semiconductors may be etched so as to have desired pixel pitches and widths in the longitudinal direction of the second stripe electrode. That is, it is divided in both the array direction of the second stripe electrode and the array direction of the first stripe electrode (longitudinal direction of the second strip electrode).
In accordance with the present invention, there is provided an image recording method of recording image information on a charge storage portion as an electrostatic latent image by irradiating a recording electromagnetic wave to the aforementioned image detector to store an electric charge with a quantity corresponding to the quantity of the electromagnetic wave in the charge storage portion, the image recording method comprising the steps of:
storing approximately uniform electric charge in the charge storage portion by applying a predetermined voltage between a first stripe electrode and a second stripe electrode; and
performing the recording, by stopping the application of the predetermined voltage and irradiating the recording electromagnetic wave to the image detector.
The expression xe2x80x9cby irradiating a recording electromagnetic wavexe2x80x9d is not limited, for example, to irradiating a recording electromagnetic wave such as radiation etc., transmitted through a subject, directly to the detector, but includes indirect irradiation in which light emitted by excitation of a recording electromagnetic wave, such as fluorescent light emitted within a scintillator, is irradiated to the detector.
Also, in the image recording method in the case of using the detector in which no scintillator is stacked, a scintillator, which emits light with a wavelength differing from the wavelength of the recording electromagnetic wave by excitation of the electromagnetic wave, may be disposed so that it faces the first electrode layer and/or second electrode layer, and light emitted from the scintillator may be irradiated to the detector. This is practically equivalent to using the detector in which a scintillator is stacked.
In this case, it is preferable to dispose a scintillator, which emits light with a wavelength differing from the wavelength of the recording electromagnetic wave by excitation of the electromagnetic wave, so that it faces one of the first and second electrode layers, and it is also preferable to irradiate the recording electromagnetic wave to the other electrode layer in which the scintillator is not disposed. This is because radiation can be utilized without waste and the charge generation efficiency can be enhanced.
Similarly, in the recording method in the case of using the detector in which the scintillator is disposed only on the outside of one of the first and second electrode layers, irradiating the recording electromagnetic wave to the electrode layer in which the scintillator is not disposed is preferred, because radiation can be utilized without waste and the charge generation efficiency can be enhanced.
In accordance with the present invention, there is provided an image reading method of reading out image information from the above-mentioned image detector in which the image information is recorded as an electrostatic latent image, the image reading method comprising the steps of:
applying a predetermined voltage between each line electrode, which constitutes a first stripe electrode, and all line electrodes, which constitute a second stripe electrode;
detecting a charging current which flows in the image detector by the application of the predetermined voltage; and
obtaining an electrical signal with a level corresponding to the quantity of an electric charge stored in a charge storage portion, by the detection of the charging current.
In accordance with the present invention, there is also provided a unit for realizing the above-mentioned image recording method, i.e., an image recorder for recording image information on a charge storage portion as an electrostatic latent image by irradiating a recording electromagnetic wave to the above-mentioned image detector to store an electric charge with a quantity corresponding to the dose of the electromagnetic wave in the charge storage portion, the image recorder comprising:
recording-voltage applying means for storing approximately uniform electric charge in the charge storage portion by applying a predetermined voltage between a first stripe electrode and a second stripe electrode; and
control means for controlling the recording-voltage applying means and the irradiation of the electromagnetic wave so that after the application of the predetermined voltage is stopped, the recording electromagnetic wave is irradiated to the image detector.
Furthermore, in accordance with the present invention, there is provided a unit for realizing the above-mentioned image reading method, i.e., an image reader for reading out image information from the above-mentioned image detector in which the image information is recorded as an electrostatic latent image, the image reader comprising:
reading-voltage applying means for applying a predetermined voltage between each line electrode, which constitutes a first stripe electrode, and all line electrodes, which constitute a second stripe electrode; and
image signal acquisition means for acquiring an electrical signal with a level corresponding to the quantity of an electric charge stored in a charge storage portion, by detecting a charging current which flows in the image detector by the application of the predetermined voltage.
When applying a predetermined voltage between each line electrode of the first stripe electrode and all the line electrodes of the second stripe electrode, it is preferred to apply the voltage in sequence in the longitudinal direction of the line element of the first stripe electrode, from one end toward the other end.
According to the image recording method and image recorder of the present invention, an electrical signal with a level corresponding to the quantity of the latent image stored in the charge storage portion is obtained, by using an image detector in which the line electrodes of the first stripe electrode are disposed so as to cross the line electrodes of the second stripe electrode, also by applying a predetermined voltage between each line electrode of the first stripe electrode and all the line electrodes of the second stripe electrode, and by detecting a charging current that flows in the detector by the application of the voltage. Therefore, a simple switch, such as a metal-oxide-semiconductor field-effect transistor (MOSFET), etc., can be used as a means for connecting each line electrode of the first stripe electrode with all the line electrodes of the second stripe electrode, and the image reader of the present invention can be constructed simply and cheaply.
Also, according to the image recording method and image recorder of the present invention, voltage is applied across the detector to store uniform electric charge in the charge storage portion, and then recording light is irradiated to store the latent image charge carrying image information in the charge storage portion. Therefore, since no light source is required as a means for storing uniform electric charge in the charge storage portion, the image recorder of the present invention can also be constructed simply and cheaply.
In addition, since the image detector used in the image recording method, the image recorder, the image reading method, and the image reader uses no thin film transistor and has a simple structure in which layers are stacked, the fabrication cost can be reduced.
Furthermore, since a scintillator can be stacked on the detector, it becomes possible to use a desired detector according to photographing conditions, such as the surface of the detector through which recording light is irradiated, the magnitude of an irradiation dose, etc. Therefore, the detectors of the present invention are convenient to use.
For example, if the rectifying layer or charge storage portion is caused to have transparency with respect to a recording electromagnetic wave or fluorescent light, radiation or fluorescent light irradiated through the upper and lower surface of the detector can be detected and therefore sensitivity can be enhanced. In addition, if a scintillator is stacked on the detector, recording radiation can be converted to fluorescent light, and this fluorescent light can be irradiated to the photoconductive layer. Therefore, the charge generation efficiency is enhanced. As a result, the signal-to-noise (S/N) ratio of an image can be enhanced and the radiation dose can be reduced to decrease a radiation dose that to which a subject is exposed.
If the charge storage portion of the detector is formed by a large number of conductive members provided separately for each pixel, sharpness during recording can be enhanced.
Also, if a charge storage portion is formed by conductive members (micro plates), which are provided separately in an electrically non-connected state for each pixel position and cause latent image charge to be in the same electric potential state, it becomes possible to cause the latent image charge for each pixel stored on the conductive members to be in the same electric potential state. Compared with the case where there is no conductive member, the charge storage portion can be satisfactorily formed and the reading efficiency can be improved. The reason for this is that since the potential of the latent image charge is held constant within the conductive member, the latent image charge which is generally difficult to read out can transfer to the central portion of the conductive member (i.e., the pixel central portion) in accordance with the process of read operation, as long as it is within the conductive layer, and therefore the latent image charge can be discharged more sufficiently.
Furthermore, if the n-type semiconductor and the p-type semiconductor (preferably, including the insulator), which form the rectifying layer, are divided as described above by etching, charge-transfer within the semiconductor can be restricted to the divided range. As a result, an image detector, which is capable of reading out latent image charge while reliably storing it at pixel positions and also capable of satisfactorily reproducing an image, can be fabricated comparatively simply.